Patients with Hermansky-Pudlak Syndrome (HPS) have minor to moderate bleeding histories. We have examined platelet function, in particular platelet aggregation and release reaction, in these patients and found that they have deficient dense granule contents. We have measured plasma and platelet von Willebrand factor, and have measured other platelet a- granule constituents such as PF-4 and b-TG in platelet lysates of HPS patients. There is significant decrease in platelet von Willebrand factor activity compared with non-HPS patients and control subjects.